The references referred to hereinafter, each of which are hereby incorporated by reference, disclose the state of the art.
Refrigeration machines of the absorption type utilizing ammonia as the refrigerant fluid and water as the absorbent, have been known for many years. These machines have been used for example in household refrigerators and air conditioners and utilize a burner of natural gas or electricity as the source of energy for heating the aqueous solution of ammonia. An example of this type of absorption heat pump, not used for cooling or as a chiller, is described in U.S. Pat. No. 4,573,330 to van der Sluys et al. This patent describes a particular design of generator (integrated to a rectifier) wherein fins are provided internally within the generator vessel to promote the formation of liquid drops. The heat is provided by a burner (63) and is not intended to operate with solar energy or other heat source of low temperature and low heat density, therefore this patent does not teach or suggest in any way a solution to the problem of transferring heat with sufficient efficiency from an energy source, such as a solar collector, to the ammonia solution in the generator.
U.S. Pat. No. 4,744,224 gives as a general background of solar driven ammonia absorption refrigerators. This patent describes an intermittent cycle system where the structure of the solar energy collector functions directly as the generator by day and as the absorber by night. The solar radiation directly heats up the ammonia solution circulating through the solar collector. Although this proposal may seem more efficient than the customary practice of heating water in the collectors and then using said hot water to heat the ammonia solution, it has several drawbacks. For example, it does not make use of commercially available solar collectors, because those commercial collectors would have to be operated at high pressures (approximately 14 bars). Also, implicitly it would require conduits for ammonia of considerable length with the consequent increase in risk of leaks. This system is also limited with respect to the temperature that can be reached by the solution and consequently also with respect to the cooling capacity of the machine as well as in its general operation. A machine operating according to this patent would not work at all if the temperature in the generator is not sufficiently high (of the order of 100.degree. C. when the ambient temperature is about 30.degree. C.); as can be derived from a pressure-temperature diagram for aqueous ammonia solutions.
U.S. Pat. No. 4,763,488 discloses a heat exchanger made of parallel plates, which can be utilized as a generator in an ammonia based solar driven absorption system. The generator structure disclosed in this patent however is of complex manufacture and also has the disadvantage of not fitting in the preferred design of existing refrigeration machines.
The present invention overcomes the disadvantages of the prior art and renders commercially possible the more effective utilization of solar energy for cooling purposes as air conditioning and ice making. One of the main reasons why the solar energy has not been yet widely applied for air conditioning, refrigeration or ice making is that the density of energy available from the sun radiation is very low, therefore, a combination of sufficiently high temperatures and energy density has to be obtained to operate a commercially available cooling machine.
The applicant has found that by combining an ammonia based absorption machine, of the type commercially available, and that has been already optimized both in its thermodynamic process and mechanical components, with a suitable solar collector and a generator modified to incorporate this invention (with little or no change in the size or bulk of the generator), the solar energy can be effectively and efficiently utilized for these purposes. This invention also makes it possible to utilize other low temperature energy sources, as for example waste heat carried by fluid streams in many industrial processes, which heat is usually thrown away to the environment, contributing to thermal pollution.
Many attempts have been made in the past in order to find an effective way of transferring the amount of necessary heat to the ammonia solution and at the same time to reach sufficiently high temperatures in the generator in order to effectively separate the ammonia from the water. The simplest way of combining a solar collector with an absorption cooling machine is to circulate water through a solar collector and then pass the hot water through heat exchange pipes in the generator where the ammonia solution contacts said pipes. This simple combination however does not work because the amount of heat per unit area to be transferred through the heat-exchange pipes can not flow through the practically-available heat transfer area under such conditions.
It is therefore an object of the invention to provide a device useful as a generator in an ammonia-absorption cooling machine which may be operated utilizing relatively low temperature energy sources, and especially which can be used with commercially available components of high-temperature driven cooling machine of the ammonia-absorption type by substituting for the burner unit yet without increasing the size of the generator core.
It is another object of the invention to provide a solar driven ammonia-absorption cooling machine.
Other objects of the invention will be pointed out hereafter or will be evident to those skilled in the art.
The objects of the invention are generally achieved by an apparatus combining a source of low temperature energy, of the type which can heat a liquid to temperatures in the range of 120.degree. C. to 150.degree. C., for example a solar energy collector, and an ammonia-absorption type cooling machine which comprises a generator vessel where an aqueous solution of ammonia is heated to separate absorbed ammonia from the solvent water by a multi-stage equilibrium separation; a hot water circulation vessel, coaxially enclosing at least a portion of and cooperating with said generator vessel to define a hot water circulation chamber through which water is caused to circulate in direct contact with the wall of said generator vessel, thus transferring heat from said hot water to said ammonia solution; characterized by the fact that said generator vessel has a multitude of heat transfer fins protruding into and preferably substantially filling said hot water circulation chamber arranged in staggered series along the wall of said generator vessel so that the hot water circulating in contact with said fins is subject to frequent direction changes resulting in heat transfer enhancing. Without intending to link the effectiveness of the present invention to the following explanation, the applicant believes that the efficiency of the heat transfer fins having the dimensions herein claimed, particularly the length, measured in the direction of the circulation of hot water through said hot water circulation chamber, is effective for this application because the many abrupt changes in direction cause turbulence breaking up establishment of thicker more steady-state boundary layers that inhibit efficient convection heat transfer at the fin surfaces. Said fin length is shorter than the length where the thick steady state boundary layers would be formed by the water circulating straight through said chamber at each side of the heat transfer fins. In other words, as the fins length is increased much beyond 50 mm there is a significent decrease in the heat transfer efficiency thereafter.